The invention relates to an antenna assembly having a retractable antenna comprising a helical antenna element carried by an elongate antenna element which may be applied, for example, to a portable radio and, in particular to a hand portable radio telephone.
Retractable antennas are of particular benefit in portable radio telephones as they enable the telephone to be conveniently carried by a user when the antenna is in the retracted position while providing appropriate operating characteristics in both the retracted and the extended positions.
U.S. Pat. No. 4,868,576 describes an antenna for a portable cellular telephone comprising a helical coil at the base of a retractable elongate radiating element. The retractable element which extends through the helical coil, has non-conductive portions at its two ends whereby the elongate element is capacitatively coupled to the helical coil, and in the retracted position the elongate element is substantially decoupled therefrom. The helical coil is fixedly mounted on the housing of the radio transceiver.
Another type of retractable antenna is described in copending patent application No. 9115134.0 entitled Retractable Antenna with Helical End. In the antenna disclosed in this application the helical element is carried by the elongate element. Antennas of this type provide for satisfactory performance with the antenna retracted for stand-by operation with better sensitivity and range performance during normal use with the antenna extended.
To provide the proper matching, the elongate element and the helical element are electrically isolated. This enables matching networks to be provided to ensure that the characteristic impedance to the feed point is the same when the antenna is retracted and extended. The matching network includes a coaxial feed that supplies both the extended and the retracted antenna, the elongate antenna providing the central conductor of the coaxial feed when in the retracted condition. In this solution there are currents flowing within the case of the portable telephone when the antenna is retracted which create stray fields that might interfere with correct circuit operation.
In addition to the unwelcome currents flowing in the case, ensuring electrical isolation between the helical and elongate element is wasteful of resources in that the additional gain that the helical portion of the antenna could provide in the extended position is not utilised. The electrical isolation would normally be created by an air gap between the two elements. This can also create a weakness in the antenna at the junction between the component elements making it less robust when extended.